Polyimide resins have Outstanding thermal resistance and solvent stability, as well as good mechanical strength and tensile properties. They are used in wide areas, such as photographic films, insulation materials for heavy electrical equipment, carrier tapes, and heat-insulating laminating materials in aircraft and spaceships. They are also used as the matrix resin in varnish, tape, adhesives and fiberreinforced composite materials.
Current research and development work attempts to create new types of polyimides that have advantages in addition to the known polyimide properties. In particular materials which can be used to make films having excellent dimensional stability and dimensional accuracy are highly desirable. Other properties which are being sought include excellent storage stability, moldability and improved impact resistance. Polyimide resins exhibiting such traits could be used for the preparation of highly selective separation membranes or heat resistant adhesives.
Generally, polyimides are made by mixing equal mols of highly pure acid dianhydride and aromatic diamine, running a polycondensation reaction in a polar solvent at low temperature to form a high molecular weight polyamic acid, followed by casting the polyamic acid solution, and finally running a dehydration/ring closure reaction by heating or by chemical treatment such as by addition of acetic anhydride ["New Heat-resistant Resins," by Lee, Stoffey and Neville, published by Tokyo Kagaku Dojin K.K., page 216].
The polyamic acid formed as an intermediate in this method is unstable upon heating. Imidization takes place along with the formation of water. The formed water is known to act on the polyamic acid, enhance hydrolysis of polyamic acid, and cut the molecular chain to form lower molecular weight polyamic acids. In order to prepare a polyimide that has high performance and high molecular weight, a method of preparation of a high molecular weight polyamic acid at room temperature or a lower temperature is generally used.
Generally, the high molecular weight polyamic acid intermediate is soluble in polar solvents such as N-methyl pyrrolidone ("NMP") or dimethylformamide and so on, but is insoluble in nearly all of other solvents. For this reason, a two-step synthetic process for preparation of polyimide via polyamic acid is often used.
On the other hand, phenolic solvents, particularly 4-chlorophenol, can dissolve a certain type of polyimides. Therefore, polyimide has been prepared in one step using this solvent, without going through the polyamic acid intermediate [Japanese Patent Publication (Kokoku) SHO 62-25405 (1987)].
And, since some polyimides are soluble in N-methylpyrrolidone, dimethylformamide, nitrobenzene and m-cresol, a high molecular weight polyamic acid can be prepared in such solvent, and then the high molecular weight polyamic acid is treated chemically to obtain a polyimide resin composition dissolved in the solvent. Particularly when cresol or nitrobenzene was used, the formed water is removed from the reaction system by azeotropic distillation to obtain a high molecular weight polyimide resin composition of a type which is soluble in the solvent [Japanese Patent Publication (Kokoku) SHO 64-1494 (1989)].
The above-described polyimides are two-component homopolymers. Physical and chemical properties of homopolymers can be regulated by the properties of the two components that constitute such homopolymers. Homopolymers having poor mechanical strength are improved by treating with homopolymers that have entirely different types of components. With the tricomponent polyimide resin composition, a deficiency in mechanical strength may be corrected by adding yet another component. Thus, the resin can be modified to suit a desired application by complementing an inferior properties while retaining its original function.
Polyimide resins made from three or more components are conventionally prepared by a random copolymerization process or by a block copolymerization process.
Normally, tri-component polyimides are prepared by a random copolymerization process where three components are mixed and polycondensed at the same time to form a resin. In Japanese Patent Publication (Kokoku) SHO 63-66852 (1988), two different types of diamines and an acid dianhydride are mixed together at the same time to form a high molecular weight polyamic acid, and then heated or chemically treated to acquire a polyimide with modified physical properties: softness, resiliency, flexibility and elongation to film or fibers. In Japanese Patent Publication (Kokai) SHO 61-296031 (1986), a tri-component random copolymer has been prepared for a purpose of improving the softness, resiliency, workability and melt flow properties. In Japanese Patent Publication (Kokai) SHO 63-314242 (1988), which discloses a method for preparation of polyimide having acceptable dimensional stability, diamine A and 40-90 mol % of acid dianhydride are reacted to prepare an amic acid prepolymer, and then 90-10 mol %, relative to the total diamine components, of diamine B is added to prepare a solution of the polyamic acid copolymer, and this solution is cast or coated to form a film which is then dried and heated or chemically treated to dehydrate/close the ring/imidize the amic acid copolymer, to form a polyimide copolymer membrane. And, in Japanese Patent Publication (Kokai), HEI 1-96220 (1989), two types of diamines are mixed with an acid dianhydride, and then they are imidized and condensed in one step to obtain a random copolymer that can serve as a copolymer resin with outstanding moldability and has an excellent balance of heat resistance and mechanical properties.
Polyimide resins made of three components or more can be prepared easily by the random copolymerization process. However, control of the final product character is not easily achieved. Particularly when a two-step poly-condensation process that goes through polyamic acid is adopted and the conventional polyimide preparation method is employed, the high molecular weight polyamic acid intermediate is unstable against heat. Hydrolysis by the formed water is enhanced to cause fission of the molecular chains, which may recondense at the imidization stage to enhance the polymer's randomness. Furthermore, it is known that 15 the high molecular weight polyamic acid intermediate can undergo exchange of acid amide groups among molecules easily and quickly. Therefore, the polyimide resins that went through a polyamic acid intermediate can be called polyimides that have a random character. This random copolymer is noted as having inferior physical and chemical properties as opposed to alternate copolymers or block copolymers. This has been attributed to the appearance of the average properties of the formed copolymer, which is inferior to the excellent properties of each of the constitutive components.
Polyimides made by a block copolymerization process might exhibit improved physical properties compared to the random copolymers. Some block polyimides have been prepared.
In Japanese Patent Publication (Kokai) SHO 60-166326 (1985), an oligomer of sulfonamide is prepared, and then this oligomer is added with an acid dianhydride to obtain a blocking polyimide resin. This resin has improved the mechanical strength, heat resistance, resistance against heating and aging and workability. In Japanese Patent Publication (Kokaku) HEI 1-21165 (1989), 1.5-2.0 mols of an acid is added to a diamine, and the reaction is run in a polar solvent at low temperature, to synthesize an oligomer of amidic acid. If an equivalent amount of isocyanate is added to carry out the reaction, a polyimideamidecarboxylic acid is obtained, while generating carbon dioxide gas. This substance is cast and then treated with heat, to prepare a polyimide film that shows improved properties. In Japanese Patent Publication (Kokai) HEI 2-91124 (1990), an acid dianhydride is added to the copolymer of diaminosiloxane to form a block copolymer of siloxane amic acid, and then an equivalent amount of diamine is added to form a polyamic acid, and finally it is treated by heat or chemically, to prepare a block copolymer of siloxane-imide. That polyimide membrane exhibits excellent bondability to the substrate. In Japanese Patent Publication (Kokai) SHO 64-16835 (1989) and Japanese Patent Publication (Kokaku) Hei 1-21165, a method of preparing a polyimide copolymer by sequential addition process is described. Excess or deficient amount of acid dianhydride is added to the aromatic diamine and they are reacted to prepare a polyamic acid prepolymer, and then a supplementary amount of diamine is added to obtain a copolymer of polyamic acid. Finally, it is treated chemically or by heating, to prepare a polyimide copolymer. Because this process goes through an intermediate polyamic acid, the exchange reaction of the formed polyamic acid occurs, and therefore, a block polymer with random properties is formed.
And, when the resin is used as a separation membrane, the polyimide separation membrane made of three components which is a random copolymer or a block copolymer will show a decrease in its selectivity when an attempt is made to improve its separating properties. On occasion, the mechanical properties may decline also.
As to the polyamides, alternate copolymers have been prepared, as illustrated in Japanese Patent Publication (Kokai) HEI 2-97527 (1990). In Japanese Patent Publication (Kokai) SHO 51-34300 (1976), an equivalent amount of diamine is reacted with polyamidic dianhydride to prepare a polyamideimide, and thus an attempt has been made to develop new applications that take advantage of the properties of block copolymers.